According to the information I have they don't recommend it. It looks like the controls are put at the bottom of the door to discourage an under counter install.

If it's a plumbed Gagg steam oven, it should be at countertop height so that the gravity drain will work, assuming that it is run to a sink drain, and the drain tubing is limited to 16'. However, a plumbed steam oven can be installed below countertop height if the drain tubing can be run straight down through the floor to a drainpipe. The controls are on the bottom of the oven, so you'll have to decide if this is too low for you.

We've had ours installed undercounter and in use for two years. We attached it to the sink drain; just made sure there was a steady incline down from the appliance. And--we're getting ready to do it again in a new kitchen. It is a bit more difficult to operate the controls since they slightly slant downward, as jakvis mentions, but fairly quickly you learn to set the controls without much need to look. This istallation set-up allowed us a nice drawer underneath to hold racks and pans.

You MUST be careful about standing back to open it...the steam could burn your face or arms as it rises.

Or remember to push the purge steam button & let it run before opening the door.